This invention is generally directed to processes for the preparation of toner compositions, primarily in situ toners. Xerographic toners exhibiting low melt properties can be fused at lower temperatures than those toners typically used in xerography, resulting in reduced energy consumption, improved reliability, lower cost and higher speed. Low melt toners can be prepared by at least two general methods. The first method involves preparation of a low melt toner composition, while the second method is based on melt mixing polymers with widely varying properties to yield a composite material with the desired properties. For example, in U.S. Pat. No. 5,229,242 there is illustrated a toner comprised of a mixture of a linear polymer, which acts as the matrix polymer, a crosslinked polymer, which is incorporated to improve fusing latitude, a wax, which is added to provide lubrication, and a copolymer compatibilizer to enable dispersion of wax in the matrix polymer. Generally, the matrix polymer is a low molecular weight polymer with a suitably high glass transition temperature that provides the required low melting behavior to the toner. The polymer in the dispersed domains is a high molecular weight polymer that provides higher elasticity and, therefore, required hot offset behavior to the toner. The dispersed phase polymer may be crosslinked. It is also possible to use a low molecular weight but highly elastic polymer for the dispersed phase, for example low molecular weight polyolefins. Low melt toners may be prepared either by a conventional toner manufacturing approach based on pulverizing a resin that has been melt blended with pigments, charge control agents and other additives, or by an in situ toner process in which the final toner particles containing all necessary pigments, charge control agents and other additives are prepared directly in a chemical reactor. Regardless of whether the toner is prepared by an in situ approach or by a conventional pulverization approach, attainment of low melt properties requires that a dispersed phase of a polymeric material exists in a continuous matrix of another polymeric material. This requirement dictates that dispersion of the minor components be of excellent quality, that is the size of the dispersed phase domains should be as small as possible, preferably less than approximately one micron in diameter. However, there is considerable difficulty in preparing resins or particles with such a microphase morphology since most polymer pairs are not compatible, blending or mixing two polymers can be difficult. Achieving a level of mixing sufficiently intensive to reduce the size of the dispersed phase domains to the range of a micron or less is extremely difficult. Methods are known for preparing well-dispersed blends of incompatible polymers, one such method involving the use of a Banbury type mixer with very high shear at relatively low temperature to provide intensive mixing. One disadvantage of the Banbury process is that it is a batch process. Batch processes are generally uneconomical. Also, extruders cannot usually be operated at the low temperatures required to attain the same effective mixing provided by Banbury type mixers. Both the Banbury mixing and extrusion processes also suffer from the disadvantage of being applicable only to the preparation of conventional toner, and not to the preparation of in situ toner. Another approach that can be used to prepare polymer blends involves use of a compatibilizer, for example a block copolymer or a graft copolymer of one type of segment compatible with the continuous phase polymer, and one type of segment compatible with the dispersed phase polymer. When the polymers and the compatibilizer are blended, the compatibilizer preferentially locates at the interfacial regions between the phases, providing reduced interfacial tension and increased phase stability. The disadvantages of relying on compatibilizers include the addition of another polymer to the system which can further complicate the behavior, the difficulty in locating an adequate compatibilizer, and the fact that compatibilizers are primarily only effective for high shear conventional toner manufacturing. For in situ toner, processes to provide extensive mixing within the particles are not believed to exist. Furthermore, there is the concern that the compatibilized dispersed phase will not perform its desired function in the same manner as when it is not compatibilized. For example, very well compatibilized wax may not be as effective a lubricant as free wax.
Several in situ toner preparation methods are known. These processes include dispersion polymerization, suspension polymerization, emulsion polymerization, and the like. Disclosed in U.S. Pat. No. 4,486,559 is the preparation of a toner composition by the incorporation of a prepolymer into a monomer/pigment mixture, followed by emulsion polymerization. In suspension polymerization processes, the pigment and additives such as charge control components are added to a monomer or comonomers prior to polymerization. Particle formation is achieved by the dispersion of the pigmented monomer or comonomers in a continuous phase, such as water, and the droplets of pigmented monomers are then polymerized to form toner particles. One advantage of these processes as compared to many other methods is the elimination of fusion mixing (Banbury/extruder) and pulverization classification processing. Nevertheless, it can be difficult with these processes to accomplish polymerization of pigmented monomer droplets in a diameter range of 3 to 25 microns with a narrow distribution of particle diameter of, for example, 1.3.
Also mentioned are U.S. Pat. No. 4,486,559, which discloses the incorporation of a prepolymer into a monomer toner mix followed by emulsion polymerization, U.S. Pat. Nos. 4,680,200 and 4,702,988, which illustrate emulsion polymerization; and 4,797,339 and 4,996,127, which disclose aggregation processes in which small primary particles are produced by emulsion polymerization, which particles can contain pigment on the surface.
Also, recited are the following U.S. Patents disclosing suspension polymerization U.S. Pat. Nos. 4,077,804; 4,601,968; 4,626,489; 4,816,366 and 4,845,007; 5,043,404 directed to semisuspension polymerization; and U.S. Pat. No. 3,954,898, which discloses bulk and suspension polymerization.
In U.S. Pat. No. 5,164,282 (Mahabadi), the disclosure of which is totally incorporated herein by reference, there are illustrated processes for the preparation of toners, and more specifically, semisuspension polymerized toner processes in which a mixture of monomer or comonomers, a polymerization initiator, a crosslinking component and a chain transfer component is bulk polymerized until partial polymerization, that is for example from about 10 to about 40 percent of monomer or comonomers, is converted to a polymer; thereafter mixing the partially polymerized product with pigments, optional charge control agents and other additives with, for example, a high shear homogenizer to form a uniform organic phase, dispersing the organic phase in water containing a stabilizing component with, for example, a high shear mixer to produce a narrow particle size toner suspension; and polymerizing the suspension product. The toner obtained can then be washed/dried and dry blended with surface flow aid additives.
However, none of these processes for the preparation of toner involve the incorporation of a microphase dispersion of a second type of polymer in the continuous phase polymer, and obtaining an effective dispersion of a minor phase in a major phase. Similarly, there are a number of processes available for preparing polymer resins for conventional toner manufacturing based on a pulverization process. Toners have been prepared generally by fusion mixing of pigments (colorants), charge control agents and other additives into thermoplastic resins to disperse them uniformly therein. In view of the high viscosity of the mixture, a considerable amount of energy is needed to achieve uniform dispersion of pigments and other additives in the toner resin. The mixture is then cooled, followed by pulverization and classification into desired particle sizes and particle size distribution. It is known that pulverization is an energy intensive step in this process. This preparation method is capable of producing excellent toners, but requires the use of several steps which are costly, energy intensive and are limited in certain respects.
In the process for producing toners by pulverization, the material must usually be fragile so as to be readily pulverized to a certain extent. Therefore, some thermoplastic resins, which are not fragile but have acceptable fusing performance, are not usually selected for the aforementioned prior art processes. Also, if the material is too fragile, it may be excessively micropulverized and, therefore, the fines portion of the particles must be uneconomically removed. These limitations become increasingly severe for smaller particle size toners. Moreover, when a material with a low melting point is employed to improve fusing performance of the toner, fusion of such material may occur in the pulverizing device or the classifier. These processes are also unable to provide a microphase dispersion of a second type of polymer in the continuous phase of the matrix polymer. Therefore, there exists a need for a process for blending polymers that enables effective dispersion in either in situ toners or toner resin particles and, more specifically, a process for preparing particles containing different types of polymer resins consisting of one or more minor phase polymers dispersed extremely well throughout the continuous major phase. This process is based on the starved feed addition of a monomer to a suspension, semisuspension, emulsion or dispersion polymerization of polymeric particles comprised of a polymer that is incompatible with the polymer to be formed by the added monomer. Therefore, the process is amenable to suspension polymerization, dispersion polymerization, semisuspension polymerization or emulsion polymerization. In one embodiment, this process comprises a particle formation step in which pigment or dye particles and charge enhancing components are included, and then starved feed addition of another monomer occurs. Starved feed addition involves adding a monomer slowly enough that secondary droplets or polymer particles cannot form or are minimized, but rather all the added monomer diffuses through the aqueous phase and into existing particles. The starved fed monomer polymerizes to provide a polymer that is incompatible with the existing particles. Furthermore, the starved feed monomer is not more hydrophilic than the existing polymer/monomer particle to ensure that the starved feed monomer diffuses into the interior of the particle and does not form a shell around the exterior of the particle. If desired, initiator, chain transfer agent or crosslinking agent can be added to the starved feed monomer. Crosslinking agent could be used to provide very high molecular weight or crosslinked domains, while chain transfer agents could provide very low molecular weight domains. While the starved feed monomer is being added, heating continues. The added monomer, after diffusing into the particle interior, will begin to polymerize, and because it is incompatible with the matrix polymer, will phase separate into microdomains. These phase separated microdomains are thus formed in situ, unlike domains created by physical blending procedures. As the monomer actively polymerizes in the presence of the matrix polymer and perhaps monomer, it is also likely that some copolymerization or grafting will occur, thereby further enhancing the stability of the microdomains. Typical sizes of these domains are 0.05 to 3.0 microns in average diameter.